A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus bending of the patterning device may occur when it is supported by clamps at its bottom and/or top surface. This bending is caused by, for example, gravity and heating effects. One way to compensate for this bending is by adjusting one or more projection system optical elements (e.g., one or more lens elements). However, introducing a so-called “field-curvature” by using an adjustable lens element may be at the cost of introducing astigmatism, that is to say the focus difference between horizontal and vertical lines of the pattern, which inherently limits the focus performance of the lithographic apparatus. In a lithographic apparatus where the patterning device is clamped at its bottom side, two heating effects occur which partially cancel each other out. This limits the astigmatism-penalty. One of the heating effects is (global) heating which causes the patterning device to expand in its entirety, and because the patterning device is supported by clamps, this causes tension at the clamps and has the effect that the patterning device becomes somewhat convex curved. The second heating effect has to do with the presence of a chromium layer at the bottom side of a mask type patterning device, which layer is less able to pass heat through to the environment than the rest of material of the mask type patterning device (mainly quartz). This causes a temperature-gradient from the bottom to the top of the patterning device, because of which the patterning device also gets somewhat convex curved (in the downward direction).
In a lithographic apparatus, the patterning device may also or alternatively be clamped at its top side. This may be desirable, for example, because the patterning device may be loaded from below and/or a larger area may be available for the clamp. The larger area advantageously enlarges the friction force available for keeping the patterning device in place during de/acceleration so as to enable possibly higher throughput. However, with topside clamping, the two above-mentioned heating effects essentially work in the same direction, that is to say both effects tend to bend the patterning device downwards. The gravity effect also bends the patterning device downwards. Therefore, the expected downwards bending of the patterning device may increase significantly if topside clamping is used for a patterning device. Trying to compensate this significant increase in bending by using, for example a lens which is adjustable for field curvature, could introduce significant astigmatism and be expensive and time consuming to design.
United States patent application publication no. US 2005/0134829 discloses a clamp to support, for example, a patterning device of a lithographic apparatus, in which the clamp is provided with a plurality of pressure zones located around a periphery of a support to hold the patterning device. Thus locally adjusted pressures can be created in order to provide a local bending moment to locally bend the patterning device. Thus the patterning device may be corrected for unevenness, unflatness, and tilting.